One of the foundations of surgery is the use of suture to re-appose tissue, i.e., to hold tissue in a desired configuration until it can heal. In principle, suturing constitutes introducing a high tensile foreign construct (looped suture) into separate pieces of tissue in order to hold those pieces in close proximity until scar formation can occur, establishing continuity and strength between tissues. Sutures initially provide the full strength of the repair, but then become secondarily reinforcing or redundant as the tissue heals. The time until tissue healing reaches its maximal strength and is dependent on suture for approximation, therefore, is a period of marked susceptibility to failure of the repair due to forces naturally acting to pull the tissues apart.
Conventional sutures provide a circular or single-point cross-sectional profile extended over the length of the suture material. Such a suture has the great benefit of radial symmetry, which eliminates directional orientation, allowing the user (e.g., physician, surgeon, medic, etc.) to not have to worry about orienting the suture during use. However, a considerable disadvantage of the currently used single-point cross-section is that it does not effectively distribute force, and actively concentrates force at a geometric point (e.g., the point at the leading edge of the circle) creating a sharp edge in the axial dimension. Under these conditions, the tissue is continuously exposed to tension, increasing the likelihood that stress concentration at a geometric point or sharp edge will cut through the tissue.
Indeed, studies of surgical closures, a most prominent example being hernia repairs, demonstrate that the majority of failures or dehiscences occur in the early post-operative period, in the days, weeks, or months immediately following the operation, before full healing can occur. Sutures used to close the abdominal wall have high failure rates as demonstrated by the outcome of hernia formation. After a standard first-time laparotomy, the postoperative hernia occurrence rate is between 11-23%. The failure rate of sutures after hernia repair is as high as 54%. This is a sizeable and costly clinical problem, with approximately 90,000 post-operative hernia repairs performed annually in the United States. Surgical failures have been blamed on poor suture placement, suture composition, patient issues such as smoking and obesity, and defects in cellular and extracellular matrices. Clinical experience in examining the cause of these surgical failures reveals that it is not breakage of suture as is commonly thought; in the majority of cases the cause is tearing of the tissue around the suture, or from another perspective, intact stronger suture cutting through weaker tissue. Mechanical analysis of the suture construct holding tissue together shows that a fundamental problem with current suture design is stress concentration at the suture puncture points through the tissue. That is, as forces act to pull tissues apart, rather than stress being more evenly distributed throughout the repair, it is instead concentrated at each point where the suture pierces through the tissue. The results are twofold: (1) constant stress at suture puncture points causes sliding of tissue around suture and enlargement of the holes, leading to loosening of the repair and an impairment of wound healing, and (2) at every puncture point where the stress concentration exceeds the mechanical strength of the tissue, the suture slices through the tissue causing surgical dehiscence. In addition, high pressure on the tissue created during tightening of the surgical knot can lead to local tissue dysfunction, irritation, inflammation, infection, and in the worst case tissue necrosis. This tissue necrosis found within the suture loop is one additional factor of eventual surgical failure.
There has been no commercial solution to the aforementioned problems with conventional sutures. Rather, thinner sutures continue to be preferred because it is commonly thought that a smaller diameter may minimize tissue injury. However, the small cross-sectional diameter in fact increases the local forces applied to the tissue, thereby increasing suture pull-through and eventual surgical failure.
One alternative to the conventional suture is disclosed by Calvin H. Frazier in U.S. Pat. No. 4,034,763. The Frazier patent discloses a tubular suture manufactured from loosely woven or expanded plastic material that has sufficient microporosity to be penetrated with newly formed tissue after introduction into the body. The Frazier patent does not expressly describe what pore sizes fall within the definition of “microporosity” and moreover it is not very clear as to what tissue “penetration” means. The Frazier patent does, however, state that the suture promotes the formation of ligamentous tissue for initially supplementing and then ultimately replacing the suture's structure and function. Furthermore, the Frazier patent describes that the suture is formed from Dacron or polytetrafluoroethylene (i.e., Teflon®), which are both commonly used as vascular grafts. From this disclosure, a person having ordinary skill in the art would understand that the suture disclosed in the Frazier patent would have pore sizes similar to those found in vascular grafts constructed from Dacron or Teflon®. It is well understood that vascular grafts constructed of these materials serve to provide a generally fluid-tight conduit for accommodating blood flow. Moreover, it is well understood that such materials have a microporosity that enables textured fibrous scar tissue formation adjacent to the graft wall such that the graft itself becomes encapsulated in that scar tissue. Tissue does not grow through the graft wall, but rather, grows about the graft wall in a textured manner. Enabling tissue in-growth through the wall of a vascular graft would be counterintuitive because vascular grafts are designed to carry blood; thus, porosity large enough to actually permit either leakage of blood or in-growth of tissue, which would restrict or block blood flow, would be counterintuitive and not contemplated. As such, these vascular grafts, and therefore the small pore sizes of the microporous suture disclosed in the Frazier patent, operate to discourage and prevent normal neovascularization and tissue in-growth into the suture. Pore sizes less than approximately 200 microns are known to be watertight and disfavor neovascularization. See, e.g., Mühl et al., New Objective Measurement to Characterize the Porosity of Textile Implants, Journal of Biomedical Materials Research Part B: Applied Biomaterials DOI 10.1002/jbmb, Page 5 (Wiley Periodicals, Inc. 2007). Accordingly, one skilled in the art would understand that the suture disclosed in the Frazier patent has a pore size that is at least less than approximately 200 microns. Thus, in summary, the Frazier patent seeks to take advantage of that microporosity to encourage the body's natural “foreign body response” of inflammation and scar tissue formation to create a fibrous scar about the suture.